You have to take different scenarios into account and think how your backup strategy will help you for each of them.
Remember that to prove that you have a good backups strategy, you have to do some restore from time to time.
The first one, is accidental deletion or corruption of some data. Your computer is working fine, but you need some files that you have deleted last month thinking you would never need them again. For that scenario, Time Machine is a good tool as long as you have enough space.
Second scenario is you have an hardware issue like a crashed hard disk and data is lost. Again Time machine is a nice tool as long as you are willing to purchase an new Mac. Time machine backup cannot be read without a Mac.
Third scenario is you have an hardware issue like a broken screen. You cannot access your data but everything is still there. Do you have another device to read the data, you will wait of a repair...
Fourth scenario: the place you are living in has burned or everything has been stolen. In that case you need an external backup. I think it can be done with time machine.
My strategy is the following.
I have a Macbook Pro.
I use time machine onto a NAS.
Weekly I do backup of the complete NAS onto an attached USB disk.
Every four weeks, I take the USB disk and put in an external site.
At this time, I bring back another USB disk from the external site it will be used in my home for four weeks.
So in case of fire, I can recover data from a few weeks ago.
Then I also try not to be linked to much to Apple products by doing the following:
- All my music is stored in the NAS. So I could use that Music with any computer able to read music files (Windows, Linux whatever).
- Regularly, I export photos from Iphotos to the NAS. Same reason as music files.
- All my e-mails, calendars and contacts are in Gmail. So I am not tied to Apple products.
I am happy with the Apple software and hardware but I do not want to be forced to by a new Mac if a disaster happen. There is a fair chance to I would decide to buy a Mac but I take precaution to ensure I could also take another decision.